Snap action pressure responsive control device with single stroke make and break

ABSTRACT

The application discloses a pressure controlled switch which includes a plunger actuated by a fluid pressure responsive diaphragm. A pair of coil springs having different spring rates oppose the movement of the plunger caused by the diaphragm. The springs are dimensioned to exert different movement resisting forces over different portions of the length of travel of the plunger and in cooperation with a spring contact structure provide low pressure dropout values, high pressure cutout values and intermediate pressure cut-in values operating the switch.

United States Patent [1 1 Krieger et al.

[451 Nov. 20, 1973 SNAP ACTION PRESSURE RESPONSIVE CONTROL DEVICE WITH SINGLE STRDKIE MAKE AND BREAK [75] Inventors: Irving W. Krieger, St. Charles; John J. Peluchiwski, Medinah; Dale F. Willcox, Aurora, all of I11.

[73] Assignee: Furnas Electric Company, Batavia,

Ill.

[22] Filed: July 9, 1971 [21] Appl. No.: 161,237

[52] US. Cl. 200/83 P, 200/67 DA, 200/83 S, 200/83 Z, 200/160 [51] Int. CL... H0111 35/34, H0111 13/38, H011! 5/20 [58] Field of Search 200/83 J, 83 SA, 200/83 S, 83 R, 83 P, 83 Z, 160, 67 DA [56] References Cited UNITED STATES PATENTS 3,110,784 11/1963 Williams et a1 200/83 P 2,739,201 3/1956 Muller 200/83 R 2,700,079 1/1955 Haydon... 200/67 DA 2,846,538 8/1958 Hagcr 200/160 X 2/1959 Williams 200/83 SA X 2,873,332 3,067,302 12/1962 Nielsen 200/83 R X 2,975,251 3/1961 Beller et a1. 200/83 P FOREIGN PATENTS OR APPLICATIONS 817,542 9/1937 France 200/83 Z 1,109,584 4/1968 Great Britain 200/83 P Primary ExaminerR0bert K. Schaefer Assistant Examiner-Robert A. Vanderhye Attorney-McDougall, Hersh & Scott [57] ABSTRACT The application discloses a pressure controlled switch which includes a plunger actuated by a fluid pressure responsive diaphragm. A pair of coil springs having different spring rates oppose the movement of the plunger caused by the diaphragm. The springs are dimensioned to exert different movement resisting forces over different portions of the length of travel of the plunger and in cooperation with a spring contact structure provide low pressure dropout values, high pressure cutout values and intermediate pressure cutin values operating the switch.

8 Claims, 12 Drawing Figures FIG 1 N VE NT 02S I w' Wkr er blzrzz lkluchl i'a ski A ancwalef Willcoz SNAP ACTION PRESSURE RESPONSIVE CONTROL DEVICE WITH SINGLE STROKE MAKE AND BREAK BACKGROUND OF THE INVENTION This invention relates generally to pressure responsive control devices. More specifically, it relates to pressure responsive electrical switches.

Pressure responsive electrical switches are used to monitor fluid pressure in closed systems and control pump motors, valves, warning lights or other elements of the system in response to pressure variations. The prior art devices with which the applicants here are familiar are constructed to operate at a given pressure. That is, the switch is in an opened or closed condition and it will operate to the opposite condition if the pressure in the system drops below a predetermined value or exceeds a predetermined value as selected by the designer. A switch of this type may, therefore, be characterized as a single pressure switch, that is, upon reaching the specified pressure the switch will operate.

Very frequently, it is desirable in fluid pressure systems effective to operate over a range of pressures. For instance, in water supply systems it may be desired to maintain fluid in the system as long as the pressure is held within a desired range. A pressure responsive switch effective to operate a single pressure may be able to effect such a result but only because in the design and manufacture of such switches tolerances may be purposely designed into its various components so that it really does operate at a single pressure. Another solution might be to provide two switches, one operating a low pressure and the other at a high pressure suitably electrically interconnected. The first solution is potentially unreliable while the second adds to the costs of control.

It is an object of this invention to provide a pressure responsive electrical switch which can be maintained in a desired operating state over a pressure range.

It is another object of this invention to provide a pressure responsive electrical switch which will operate over a desired range of pressure and which at the same time is reliable and economical to construct.

It is still another object of this invention to provide a pressure responsive electrical switch which may operate over a range of pressures which range may be adjusted by the user thereof.

SUMMARY OF THE INVENTION The foregoing and other objects are achieved in one embodiment of the invention by providing a pair of force exerting means acting against the force exerted on a contact actuating plunger by a pressure responsive movable diaphragm. The force exerting means are arranged to exert their force over difierent portions of the travel of the plunger so as to provide two distinct operating pressure values.

BRIEF DESCRIPTION OF THE DRAWINGS The invention itself is set forth in the claims appended hereto and forming a part of this specification while an understanding of the structure and operation of various embodiments thereof may be had by reference to the detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a plan view of a housing incorporating a first embodiment of a switch in accordance with the invention;

FIG. 2 is a side view of the housing of FIG. 1;

FIG. 3 is a side view of the housing of FIG. I taken at right angles to the side view of FIG. 2;

FIG. 4 is a bottom view of the housing of FIG. 1;

FIG. 5 is a view of the embodiment of FIG. 1 with the bottom of the housing, diaphragm and plate removed;

FIG. 6 is a view similar to FIG. 5 with the electrical contacts removed;

FIG. 7 is a sectional view along the lines 7-7 of FIG.

FIG. 8 is a partial sectional view along the lines 88 of FIG. 1;

FIG. 9 is a partial sectional view along the lines 99 of FIG. 5;

FIG. 10 is a perspective illustration of a contact blade usable in the invention;

FIG. 11 is a sectional view similar to the sectional view of FIG. 7 of an alternative embodiment of the invention; and

FIG. 12 is a schematic diagram of a switch circuit embodied in the invention.

DETAILED DESCRIPTION Referring first to FIGS. 1 through 3 of the drawings, a switch in accordance with the invention comprises a housing 2 which may be formed by molding a suitable plastic material having desired physical properties of mechanical strength, electrical resistance and the like. The housing comprises a base or lower portion 6 which may have a circular configuration as shown in the drawing. Extending outwardly from the base 6 are a plurality of integrally formed projecting portions 8 which receive retaining screws as will be described subsequently. Projecting upwardly from the base 6 is a hollow tubular portion 10 open at its upper end to receive an adjusting set screw 12.

Extending upwardly from the base portion 6 are a pair of electrical terminals 14 and 16. The terminal 14 may be secured to the housing by means of a rivet 18 which, as will be explained subsequently, may also be used to form part of the electrical circuit. If desired, an upstanding boss 20 may be provided on the housing and used for the purpose of providing a point where electrical conductors forming a part of the circuit may be connected to each other. This may be accomplished by providing a pressed fit threaded metal insert in the boss 20 to receive a screw which when tightened will hold two electrical conductors firmly in position and maintain electrical contact between them.

In the embodiment of the invention illustrated, the housing is provided with an opening in the upper surface of the base 6 to receive an electrically insulating push button 22 which may be used to operate a reset circuit as will be described subsequently.

The housing is substantially enclosed by providing a Referring now to FIGS. 5 through 9, the internal structure of the switch will now be explained. As may be seen, the threaded extension includes passage 32 open at one end and partially closed by an orifice 34 at its other end. If desired, the orifice 34 may be eliminated or its dimensions varied for its purpose is to establish a response time for the operation of the switch. Thus, with no orifice in the opening 32 the switch will tend to respond to pressure changes fairly rapidly. However, with an orifice the response will be slower as time will be required for fluid to pass through the orifice and build up pressure within the housing. By providing an orifice the switch can be made to respond to pressure changes which are persistent but it will not respond to those which are transitory in nature and switch operation is not desired.

Secured between the diaphragm enclosure 24 and the bottom of the housing 6 is a flexible diaphragm 38. The diaphragm is formed of a material which is flexible and relatively impervious to the fluid in the system. Resting on the surface of the diaphragm is a plate which has for its purpose spreading out over the surface of the diaphragm the load exerted thereon by a plunger 42. The plunger 42 is provided with a bottom necked down portion 44 extending from an intermediate portion 46 of slightly greater diameter. Disposed above the intermediate portion 46 is an upper portion 48 provided with a socket 50 extending open from its upper surface 52 encompassing the socket.

A first force exerting means comprises a coil spring 54 having its bottom resting on the plunger surface 52 while its top bears against a disc 56 backed up by the set screw 12. The disc 56 is provided with a raised center portion 58 providing a positioning or locating means for a second force exerting means comprising a coil spring 60, the lower end of which extends into the socket 50.

The plunger structure is completed by a tubular sleeve 62 which is movably mounted on the necked down plunger portion 44 and rests on the plate 40. The upper portion of the sleeve 62 is provided with a surface sloping inwardly and upwardly toward the plunger portion 44 for a purpose to be described hereinafter. As may be seen, the bottom of the sleeve is provided with a similar sloping surface 66 so that when the switch is being assembled the assembler need not be concerned about which end of the sleeve is inserted over the portion 44 first. A similar sloping surface 68 is provided on the intermediate plunger portion 46.

The structure described to this point constitutes the mechanical actuating portion of the switch assembly. As may be best seen in FIG. 7, when fluid under pressure enters the diaphragm enclosure through the passage 32 and orifice 34 the pressure on the diaphragm acting on the bottom of the sleeve 62 forces it to move in an upward direction as shown in the drawing. When the sleeve has moved to the point where its surface 64 contacts the surface 68 of the plunger portion 46 further it is first opposed by the spring 60 which is selected to have a first and relatively low spring rate. The length of the spring 60 is selected to be such so that over the first portion of the upward travel of the plunger in response to the force exerted by the diaphragm it alone exerts an opposing force. As the fluid pressure in the diaphragm enclosure increases, the surface 52 of the plunger portion 48 engages the bottom of the spring 54. The spring 54 is designed to have a second and relatively high spring rate and to have an uncompressed length such that it exerts no force on the plunger until the plunger has traveled upwardly a predetermined amount. Thus, the upward movement of the plunger will be opposed first by the spring 60 and then by the spring 54. By rotating the set screw 12 so as to force the disc 56 downwardly spring forces may be adjusted although in the embodiment illustrated the difference between the two forces will remain constant.

An electrical switch to be operated by the plunger is constituted by a movable contact blade 70 which is mounted in the housing with one end 72 fixed by means of a screw 74 passing through an opening 76 in the base 6 of the housing and threaded into the base of the terminal 16 as shown at 78.

The contact blade 70 comprises a center leg 80 and a pair of side legs 82. The contact switch blade is formed of suitable electrically conducting spring material. The free end of the contact blade carries an elec trical contact element 84. A blade stop 86 is secured to the housing by means of a screw 88 engaging in a threaded boss 90 formed as part of the housing and comprises an arcuate blade, the free end of which engages under the free end of the contact blade to limit its downward movement.

As may be seen in FIGS. 7 and 10, the outer legs 82 of the spring contact blade 70 exert a force tending to bow the center leg of the blade downwardly as shown in the drawing. As the center leg moves upwardly in response to increased pressure in the system that leg tends to straighten out or elongate until a point where the center leg starts to bow upwardly and the free end of the blade moves downwardly with a snap action.

The switch construction is completed by providing a fixed contact 92 which is constituted by the lower end of the rivet 18 holding the terminal 14 on the housing. If desired, a reset switch may be provided utilizing the reset switch button 22. The reset circuit includes a movable switch arm 94, one end of which is held in position by means of the rivet contact 18. The reset circuit is completed when the switch arm 94 is depressed by the reset button 22 and engages a fixed arcuate contact arm 96 held in position by the screw 74 mounting the switch contact blade 70.

The electrical circuit thus provided may be seen in FIG. 12. In this figure, a terminal 14 is connected to the fixed contact 92 and the switch arm 94. The switch blade 70 provided with the movable contact 84 is connected to the other terminal 16. Also connected to the terminal 16 is the stationary contact 96 which may be engaged by the movable switch arm 94. Thus, a circuit from terminal 14 to 16 may be completed by actuation of either switch 70 or 94.

In the operation of this embodiment of the invention the arrangement of the parts is such that the switch is first open when the fluid pressure in a system is below a preset value. As pressure in the system increases, that pressure is transmitted through the passage 32 and orifice 34 so that it is exerted against the bottom of the diaphragm 38.

As the pressure in the fluid system increases, the diaphragm is forced upwardly and exerts an upward force on the sleeve 66. This upward force is applied to the bottom of the contact blade 70 and forces the center leg 80 of the spring contact blade 70 upwardly. The sloping surfaces 64 of the sleeve and 68 of the plunger provide space to permit relative movement between the center leg 80 and the plunger portion 44. The upward motion of the center leg causes the contact 84 to engage the contact 92 completing an electrical circuit from the terminal 16 through the spring contact blade 70, movable contact 84, fixed contact 92 and rivet contact 18 to the terminal 14.

The parts are dimensioned such that a first and relatively low value of fluid pressure will cause the circuit to be closed. In a particular embodiment, this value of pressure may be selected to be 1 pound per square inch. As the fluid pressure increases, the plunger will continue to move upwardly until its surface 52 engages the lower portion of the coil spring 54. When this point is reached, further movement of the plunger will be resisted by the force exerted by the coil spring 54 and unless the fluid pressure force exceeds that force the plunger will move no further and contact will be maintained. Upon a further increase in pressure in excess of that exerted by the spring 54 the plunger will move upwardly permitting the sleeve acting on the bottom of the contact blade to move it upwardly causing it to move pass the point where it bows upwardly so that its free end moves downwardly with a snap action.

At this point the circuit previously described is interrupted at a value of pressure which may be determined by the parameters of the components to be considerably higher than the pressure at which the circuit was first closed. Thus, there is provided a device which may control an electrical circuit so as to operate the circuit to a first condition at a first and lower value of pressure and to operate that circuit to a second condition at a second and higher value pr pressure. In the embodiment illustrated, the circuit is energized or completed at the first value and de-energized or interrupted at the second value.

The opening and closing of the circuit can, of course, be used to effect any desired control function. For instance, the switch may be connected to a fluid pumping system so as to respond to the pressure maintained by a pump supplying fluid from a reservoir or other source. Electrical connections may then be made from the terminals 14 and 16 to an electric motor driving the pump in such a way as to cause the pump to be deenergized in the event the fluid pressure exceeds a predetermined value.

In the further operation of the system, when the circuit is interrupted as the result of the pressure exceeding the predetermined value fixed by the spring constant of the coil spring 54 and the pressure then drops, for instance, because the switch is used to control the energization of the motor driving a pump supplying this system, the pressure in the system will then drop. At this point the coil spring 54 exerts a downward pressure on the plunger 42 causing it to move in the downward direction. As the spring blade contact moves downwardly due first to the action of the coil spring 54 and then to the action of the coil spring 60 there is a brief moment when the contacts 84 and 92 once again engage each other. However, this contact may be so momentary that in a normal situation it will not be effective to permit the motor to be energized sufficiently long to permit the pump to once again build up pressure in the system to its desired value. In order to once again restore pressure to the system, the reset button 22 may be operated manually. Upon depression of the reset button the spring contact blade 94 is forced downwardly to complete an electrical circuit through terminal 14, contact rivet 18, movable contact blade 94 and contact arm 96 to the terminal 16. By holding the reset button 22 down a circuit will be maintained permitting the pump motor to be energized and the pump once again to operate to force fluid through the system. When the pressure has once again reached the desired minimum value, the switch operates in the manner previously described to cause contact 84 to engage contact 92 permitting the reset button to be released and the pressure responsive switch in accordance with the invention to once again take control.

Obviously, the application for a switch in accordance with the invention just described is only one to which it may be put. For instance, the switch could be utilized in circuits controlling alarms or other means such as lights indicating whether or not the pressure in the system is above or below desired values. Alternatively, it may be used to control devices external to the fluid system itself wherein it is desired to control such a device in response to pressure in the system.

In the embodiment of the invention just described it has been found that the particular arrangement of the coil springs 60 and 54 is most advantageously used when it is desired to construct a device responding to pressures which are relatively low. When it is desired to construct a device which responds to somewhat higher pressures, the embodiment of FIG. 11 may be used.

In FIG. 11, as may be seen, the construction of the switch, insofar as the diaphragm, plate, spring contact and the contact structure are concerned, is the same as that of FIG. 7, and accordingly, the same reference numerals have been applied to those parts which correspond identically to the elements of FIG. 11.

The plunger 42' in the embodiment of FIG. 11 is provided with a lower extension 44 encompassed by a movable sleeve 62. Extending above the upper portion 48 of the plunger is a first necked down portion 49' and extending above that is a second and further necked down portion 51. A flattened head 53' is guided within a socket 55. formed in a generally capshaped disc 56.

A coil spring 60' is mounted so that its lower end bears against a flange 57 extending circumferentially around the capshaped disc 56' while its upper end bears against the inner surface of the hollow tubular portion 10. A second coil spring 54' is mounted between the upper surface of the plunger position 48' and a collar or washer 61 on the plunger portion 51 so as to be slightly compressed.

In the quiescent state the plunger is maintained in its lowennost position by virtue of the force exerted by the coil spring 54'. When the switch is connected into a fluid system so as to respond from the pressure therein, an increase of the fluid pressure causes the plunger to move upwardly. Initially, the upward motion of the plunger is resisted by the coil spring 60' bearing on the flange 57' which in turn bears on the washer 61'. Inasmuch as the coil spring 54' is selected to have a relative high spring rate, the force exerted by the spring 60' is transmitted to the upper surface of the plunger portion 48' so as to provide initial resistance to the upward movement of the plunger. However, when the fluid pressure is sufi'icient to overcome that resistance to movement the plunger moves upwardly, carrying with it the disc 56', until the top of that disc contacts the bottom of the set screw 12. If the pressure continues to increase to a value where it overcomes the force exerted by the spring 54', the plunger will move upward compressing that spring still further. Room for further movement of the plunger is provided by the socket 55' provided in the disc 56' into which the head 53 may move.

As in the operation of the previous embodiment, the contact blade closes the circuit a first and relatively low value of pressure and opens it when the pressure exceeds a second and a relatively high value. As the pressure decreases, the plunger moves downwardly in response to the force of the coil spring 54. The downward movement of the plunger permits the spring contact blade 70, the center leg of which had been bowed upwardly, to snap downwardly by virtue of the spring action of the outer arms 82 closing the contacts 84 and 92. If the pressure in the system during the period while the contacts 84 and 92 had been opened as the result of the upward motion of the plunger for a period sufficiently long enough for that pressure to drop below the dropout value, then the plunger will continue to move downwardly causing the contacts once again to open. Under these circumstances, it may be necessary to depress the restart button in order to once again activate the pump to cause pressure build-up in the system to the desired value.

Obviously, numerous modifications can be made in the structure of the embodiments illustrated and described, and it is intended by the claims to cover all such modifications as come within their scope.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. A pressure responsive device comprising a housing having an opening for permitting the entry of fluid into the housing, a movable diaphragm mounted in said housing dividing said housing into a first portion and a second portion receiving fluid entering through said opening, a stationary contact mounted on said housing in said first portion, a movable contact blade formed of spring material fixedly mounted at one end thereof to said housing within said first portion thereof with its free end extending toward said stationary contact and provided with a movable contact at its free end, said movable contact having a center leg and a pair of outside legs, said outside legs bent so as to exert a force normally causing said center leg to be bowed in a direction away from said stationary contact, a plunger mounted in said first portion of said housing and having a first end and a second end, said first end of said plunger extending toward said diaphragm and bearing against said movable contact blade whereby movement of said diaphragm in response to variations of fluid pressure in said housing exerts a force on said plunger to cause it to move, a first spring mounted in said housing and extending between said second end of said plunger and a wall of said housing exerting a force of a first magnitude on said plunger over a first portion of and greater magnitude on said second end of said III plunger over a second portion of its movement opposing the force exerted by said diaphragm, said first spring normally urging said movable contact blade away from said stationary contact, said diaphragm upon movement in response to the pressure of fluid entering through said opening causing said plunger and said movable contact blade to be moved against the force of said first spring and causing said movable contact to engage said stationary contact, further movement of said diaphragm in response to increased fluid pressure acting thereon causing said plunger to move further until its further movement is limited by the force exerted by said second spring and said center leg is bowed in the direction of said stationary contact to exert a force on the said free end of said movable contact to cause it to separate from said stationary contact.

2. A pressure responsive device as set forth in claim 1 wherein said center leg is provided with an opening therein, said first end of said plunger passes through said center leg opening, and a sleeve axially movable on and encompassing said first end of said plunger on the side of said center leg facing said diaphragm and normally engaging said diaphragm to transmit motion of said diaphragm to said center leg.

3. A pressure responsive device as set forth in claim 2 wherein said first spring is constituted by a coil spring and has a first and relatively low spring rate and said second spring is constituted by a second coil spring having a second and relatively high spring rate and said second spring surrounds said first spring.

4. The pressure responsive device of claim 3 including an adjustable means comprising a disc having a first surface engaged by said springs and a second surface and a set screw threaded in said housing engaging said second surface of said disc.

5. The pressure responsive device of claim 4 including a reset switch comprising a movable switch arm electrically connected to said stationary contact, a fixed contact arm electrically connected to the fixed end of said movable contact blade and a reset button engageable with said movable switch arm to cause it to engage said fixed contact arm.

6. The pressure responsive device of claim 2 wherein said second end of said plunger is provided with a reduced diameter portion, said second spring encompasses said reduced diameter portion and said first spring is mounted above said second spring.

7. The pressure responsive device of claim 6 including a cap-shaped disc interposed between said first and second springs and wherein one end of said second spring bears against said cap-shaped disc and a set screw threaded in said housing engaging a surface of said cap-shaped disc.

8. The pressure responsive device of claim 7 including a reset switch comprising a movable switch arm electrically connected to said stationary contact, a fixed contact arm electrically connected to the fixed end of said movable contact blade and a reset button engageable with said movable switch arm to cause it to engage said fixed contact arm.

* 4 it l 9K 

1. A pressure responsive device comprising a housing having an opening for permitting the entry of fluid into the housing, a movable diaphragm mounted in said housing dividing said housing into a first portion and a second portion receiving fluid entering through said opening, a stationary contact mounted on said housing in said first portion, a movable contact blade formed of spring material fixedly mounted at one end thereof to said housing within said first portion thereof with its free end extending toward said stationary contact and provided wiTh a movable contact at its free end, said movable contact having a center leg and a pair of outside legs, said outside legs bent so as to exert a force normally causing said center leg to be bowed in a direction away from said stationary contact, a plunger mounted in said first portion of said housing and having a first end and a second end, said first end of said plunger extending toward said diaphragm and bearing against said movable contact blade whereby movement of said diaphragm in response to variations of fluid pressure in said housing exerts a force on said plunger to cause it to move, a first spring mounted in said housing and extending between said second end of said plunger and a wall of said housing exerting a force of a first magnitude on said plunger over a first portion of its movement opposing the force exerted by said diaphragm, a second spring mounted in said housing and extending between said wall of said housing and said second end of said plunger exerting a force of a second and greater magnitude on said second end of said plunger over a second portion of its movement opposing the force exerted by said diaphragm, said first spring normally urging said movable contact blade away from said stationary contact, said diaphragm upon movement in response to the pressure of fluid entering through said opening causing said plunger and said movable contact blade to be moved against the force of said first spring and causing said movable contact to engage said stationary contact, further movement of said diaphragm in response to increased fluid pressure acting thereon causing said plunger to move further until its further movement is limited by the force exerted by said second spring and said center leg is bowed in the direction of said stationary contact to exert a force on the said free end of said movable contact to cause it to separate from said stationary contact.
 2. A pressure responsive device as set forth in claim 1 wherein said center leg is provided with an opening therein, said first end of said plunger passes through said center leg opening, and a sleeve axially movable on and encompassing said first end of said plunger on the side of said center leg facing said diaphragm and normally engaging said diaphragm to transmit motion of said diaphragm to said center leg.
 3. A pressure responsive device as set forth in claim 2 wherein said first spring is constituted by a coil spring and has a first and relatively low spring rate and said second spring is constituted by a second coil spring having a second and relatively high spring rate and said second spring surrounds said first spring.
 4. The pressure responsive device of claim 3 including an adjustable means comprising a disc having a first surface engaged by said springs and a second surface and a set screw threaded in said housing engaging said second surface of said disc.
 5. The pressure responsive device of claim 4 including a reset switch comprising a movable switch arm electrically connected to said stationary contact, a fixed contact arm electrically connected to the fixed end of said movable contact blade and a reset button engageable with said movable switch arm to cause it to engage said fixed contact arm.
 6. The pressure responsive device of claim 2 wherein said second end of said plunger is provided with a reduced diameter portion, said second spring encompasses said reduced diameter portion and said first spring is mounted above said second spring.
 7. The pressure responsive device of claim 6 including a cap-shaped disc interposed between said first and second springs and wherein one end of said second spring bears against said cap-shaped disc and a set screw threaded in said housing engaging a surface of said cap-shaped disc.
 8. The pressure responsive device of claim 7 including a reset switch comprising a movable switch arm electrically connected to said stationary contact, a fixed contact arm electrically connected to the fixed end of said movable contact blade and a rEset button engageable with said movable switch arm to cause it to engage said fixed contact arm. 